December 27, 2010 3 Comments
Frequency hopping is a good way to increase the number of WSPR spots, especially on bands that appear to be closed. Another way to increase the number of spots is to deploy low-cost crystal-controlled receivers and transceivers. I have a couple of crystal-controlled Softrock receivers, one for 7MHz and the other for 14MHz. The 14MHz one does not cover the WSPR band, but the 7MHz one does. I decided to give it a try.
One approach is to use PowerSDR or SDR-Radio to do IQ SSB demodulation, output the demodulated audio to a virtual audio cable (VAC), and feed the VAC audio to WSPR. But since WSPR can now do IQ demodulation itself, I thought I should be able to do it without a separate program to demodulate the audio and without VAC.
From my experiments with the Si570 Softrocks, I knew that the trick would be to set the frequency-offset parameter in WSPR (this parameter is called Fiq in the GUI). There was already some discussion of how to set it on the Softrock users’ group, but as far as I could tell nobody reported success.
One evening of experimentation resulted in a failure. One difficulty is that with WSPR you only see a 200Hz portion of the spectrum, so if you are off the mark for some reason, you simply don’t see any WSPR signals (or no signals at all). The next evening I decided to examine the sources. From the sources I discovered a formula for setting Fiq, based on the center frequency of the receiver. I gave it a try but didn’t see any signals. I did not measure the center frequency very accurately, so I knew I could be off. I moved Fiq by 100Hz and tried again. Success! I could see signals. But from comparing my reported frequencies to other reports on WSPRnet, I realized I was still around 15Hz off. I reduced Fiq by 15Hz and my reports matched those on the web site. Joe Taylor will include the formula in the manual of the upcoming WSPR 3.0.
Another issue that you need to work out is whether your IQ signal needs to be reversed. On the current beta version of WSPR, if you use the “Left/Right=Q/I” in Rocky then you don’t need to check “Reverse I/Q” in WSPR.
I left the receiver on for 5 hours (more precisely, until my laptop’s battery ran out) and spotted a large number of stations (343 spots, ranging from Japan to Venezuela).
If I can find an antenna, sound card, and computer for this receiver, I can keep it on all the time. I’ll see if I can do it with a low-power computer; I prefer not to waste hundreds of watts on this.
I think that crystal control can also be used on 10MHz Softrock receivers and on 7MHz and 10MHz crystal controlled transceivers from both the Softrock line (like the 6.2 RXTX) and the GenesisRadio line. I believe that the center frequencies of the 14MHz and 3.5MHz crystal-controlled radios from these lines are too far from the WSPR frequencies to work; I didn’t check the 1.8MHz frequencies.